6 Lymphocyte Activators and Antigen Recognition 3

Wholly responsible for the specific immune recognition of pathogens (initiate adaptive immune responses

All lymphocytes are derived from bone marrow

T lymphocytes – thymus B lymphocytes – bone marrow

Spleen Lymph nodes Appendix Tonsils Other Mucosal associated lymphoid tissues (MALT)

Recognizes a particular Ag using a receptor molecule on its surface

Different types and have a variety of functions: 1. Regulating immunoglobulin production (TH)

2. Interacting with mononuclear phagocytes and

helping them destroy pathogens (TH)3. Lysing virally infected cells ( T Cytotoxic cells-Tc)

4. Delayed hypersensitivity

Mononuclear phagocytes with cytotoxic ability

Have the capacity to recognize the surface changes in tumor cells and virally infected cells

Use recognition systems which are non-specific

NK cells

FUNCTION T cell Phenotype (65%-35%)

T1+ T3+ T4+ T11+

T1+ T3+ T8+ T11+

Effector cells for delayed Hypersensitivity

Effector cells for cytotoxicity

Help for immunoglobulin Synthesis

Help for cytotoxicity

Suppressor for Ig synthesisand delayed hypersensitivity

Inducer of suppressor

+

-

+

+ -

+

-

+

-

-

+

-

Stimulation of lymphocytes by antigen or mitogens (substances which caused lymphocytes to undergo cell division)

Followed by proliferation and differentiation of the progeny into various effector and memory cells.

3 Major Categories of Molecules that Trigger the Activation of Lymphocytes

1) Monoclonal activators – antigens (Ag)

2) Oligoclonal activators- superantigens

3) Polyclonal activators - mitogens

Antigens Stimulate/activate T cells or B cells

after contact with an antigen through their receptor specific for an epitope.

Result: generation of one clone of T cell and B cell population.

T-cell dependent Ag Antigens (Proteins) will only activate B

cells in the presence of T cells (and the proteins that they secrete).

T cell recognize the antigen cytokines are released B cells are stimulated

T- cell independent Ag B cell directly recognizes the antigen

(Polysaccharides ) Antigens that can activate B cells in the

absence of T cells

Superantigens Molecules typically derived from bacteria

Activate a subset of T cells but not all T cells

T cell recognizes particular amino acid sequences (epitopes) that are common to its receptor.

Result: activated cells generate a few, different clones

Mitogens Epitopes are recognized virtually by all

T cells and B cells Result: whole populations of T cells and

B cells are activated and many clones are generated

Plant proteins – polyclonal activators Phytohemagglutinin (PHA) and

Concanavalin A (Con A) are plant glycoproteins activate T cells

Lipopolysaccharides (LPS) activate B cells

Pokeweed mitogens (PWN) activate both T cells and B cells.

Polymorphonuclear leukocytes (PMN): basophils, neutrophils and eosinophils

Monocytes Macrophages Natural Killer cells Antigen presenting cells (APC) –

dendritic cells and macrophages Ag recognition is non-specific

Direct recognition – immediate encounter and recognition of a cell Ag is interacting directly with a receptor

present in the cell. Examples:

Phagocytes recognize a bacterium phagocytes have primitive pattern

recognition receptors that interact with

many microorganisms.

Indirect recognition another molecule (opsonin) binds

to the antigen Opsonin serves as a link between

the cell and the antigen. Examples:

Phagocytes that express receptors for opsonins

Eosinophils that express receptors for IgE antibodies that bind a helminth

Natural killer cells that express receptors for IgG antibody that bind a virally infected cell.

T cells and B cells have antigen-recognizing receptors that bind antigen but interact only with the epitopes.

CellsRecognize epitopes on

Antigen recognizing receptor

B cells Intact antigenMembrane-bound antibodies

T cellsAntigen fragments bound to MHC

T cell receptor (TCR)